This invention relates to a charcoal adsorber system for selectively holding up such fission gases as xenon and krypton included in off-gas discharged from a nuclear reactor plant. The present invention especially relates to a method and system for regenerating dehumidifiers, which dehumidifiers are provided in the charcoal adsorber system in order to dehumidify the off-gas to be treated.
During operation of a nuclear reactor plant, many kinds of materials are formed through nuclear fission processes. Among those materials are xenon and krypton, which are non-condensable radioactive noble gases, so that before discharging the off-gas to the atmosphere, it is necessary to recover and store the off-gas, such as in a charcoal adsorber system, until the radioactivity of the off-gas decreases to a certain degree so that it will not cause impermissible air contamination. By the charcoal adsorber system, radioactive noble gases are selectively kept for a certain period of time in a charcoal bed until radioactivity is effectively decayed. Meanwhile, between the relative humidity of the off-gas to be treated and the moisture content of the charcoal, there exists a certain relationship and to attain a higher adsorption efficiency of the charcoal, it is required to maintain the off-gas to be passed through the charcoal in a dry state. Usually, moisture in the off-gas to be treated is removed beforehand by passing the off-gas through a dehumidifier which includes a molecular sieve or some other desiccant. However, because of the fact that the dehumidifying ability of the dehumidifier also decreases along with the amount of moisture absorbed in the desiccant, periodic regeneration of the dehumidifier is important. In order to accomplish the regeneration of the dehumidifier, it is known to use a part of the off-gas, which yet includes radioactive noble gases, as regeneration gas. That is, according to the examined Japanese patent application, Publication No. 48-30120, published Sept. 17, 1973, a part of the off-gas which is untreated and includes radioactive noble gas, is introduced to a dehumidifier in a heated state. The heated off-gas removes moisture from the desiccant, which moisture is then separated from this off-gas by some cooling method. The off-gas employed for regeneration is recovered and returned to the charcoal adsorber system for further treatment. Normally, several hundred Nm.sup.3 /hour (normal cubic meters/hour) of regeneration gas is required to complete the regeneration within a desired period of time. However, when the amount of off-gas, which is usually small (about 1 to 10 Nm.sup.3 /h) and variable because the amount is determined by the amount of air captured by leakage in a main condenser of the nuclear plant, becomes smaller than usual, the amount of off-gas for the regeneration becomes insufficient to complete the regeneration of the dehumidifier within a desired period of time. Also, there is another drawback in the above-described regeneration system in that further contamination in the regeneration system can be expected because the radioactive off-gas is employed for the regeneration.
An object of the invention is to provide a method and system for regenerating a dehumidifier for use in a charcoal adsorber system in which the regeneration can be performed in a shorter period of time and independently of the amount of off-gas produced.
Another object of the invention is to provide a method and system for regenerating a dehumidifier for use in a charcoal adsorber system in which the regeneration can be performed without causing any radioactive contamination in the regenerating system.
According to preferred embodiments of the present invention, a method and system for regenerating a dehumidifier for use in a charcoal adsorber system is provided in which the dehumidifier which absorbs moisture from off-gas to be treated in a charcoal adsorber is purged of radioactive materials by fresh purge air and thereafter regenerated by fresh regeneration air. The fresh purge air which may be contaminated by radioactive materials within the dehumidifier is treated in the charcoal adsorber so that contamination of the dehumidifier by radioactive material during regeneration is advantageously prevented. Further, regeneration by fresh regeneration air advantageously reduces the time required for the regeneration as compared with regeneration performed by off-gas. The regeneration of the dehumidifier by fresh regeneration air is preferably performed by circulating the fresh regeneration air within the dehumidifier while heating the fresh regeneration air and separating moisture from the fresh regeneration air. The circulation of the fresh regeneration air advantageously eliminates the unnecessary increase of the amount of off-gas which should be treated in the charcoal adsorber system.
According to other preferred embodiments of the present invention, the pressure of the fresh regeneration air is controlled to be a desirable amount so that pressure changes will not cause leakage of the removed water or the fresh regeneration air during the regeneration process.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.